Jailed Russian opposition leader Aleksei Navalny’s health has rapidly deteriorated and he could suffer cardiac arrest “any minute,” according to doctors demanding immediate access to the prominent Kremlin critic.

The plea came from Navalny’s personal doctor, Anastasia Vasilyeva, and three other doctors, including a cardiologist, in a letter to Russia’s Federal Prison Service officials that was posted to Vasilyeva’s Twitter account on April 17.

Navalny, 44, announced a hunger strike at the end of last month in protest at what he said was the refusal of prison authorities to allow him to receive proper medical care for acute back and leg pain.

The opposition leader said on April 16 that prison authorities were threatening to put him in a straightjacket to force-feed him.

The doctors’ statement said that blood tests showed that Navalny’s potassium count had reached a “critical level.”

“This means both impaired renal function and that serious heart rhythm problems can happen any minute,” the letter said.

Navalny was arrested in January on his arrival from Germany where he was treated for poisoning in Siberia with what was defined by European labs as a nerve agent in August last year. He has accused President Vladimir Putin of ordering the poisoning, which Kremlin has denied.

A Moscow court sentenced the opposition leader in February to 2 1/2 years in prison on charges he says were politically motivated.

With reporting by AFP